1. Field of the Invention
The present invention relates to an avalanche photodiode and more particularly to a fast, highly sensitive, wideband avalanche photodiode with a large gain for use in optical communication.
2. Description of the Related Art
The avalanche photodiode is a light receiving device with a built-in function of amplifying an optical signal and, because of its high sensitivity and fast operation, has found a wide range of applications as an optical communication light receiving device. The amplification function of the avalanche photodiode is realized by taking advantage of an avalanche breakdown phenomenon that occurs in semiconductors. A principle by which an amplification occurs during the avalanche breakdown is briefly explained as follows.
Electrons or holes moving in a semiconductor are scattered by a crystal lattice when they strike it. Applying a large electric field to the semiconductor accelerates these carriers, resulting in an increase in their moving speed. As the moving speed of the carriers in the semiconductor increases and their kinetic energy is higher than a bandgap of the semiconductor, a probability of breaking bonds of lattice increases when they hit the crystal lattice, newly creating free-moving electron-hole pairs. An atom with its bonds broken loses electric charges and looks as if it is ionized. This phenomenon is therefore called an impact electrolytic dissociation or impact ionization, and a measure of how many electron-hole pairs are generated by the impact ionization after an electron or hole has traveled a unit distance is also called an ionization rate. A ratio of an ionization rate based on electrons to an ionization rate based on holes is further called an ionization rate ratio.
Newly created carriers (electrons or holes) produced by the impact ionization are also accelerated by the electric field and acquire a kinetic energy, with subsequent impact ionizations further creating new carriers. As the impact ionization repetitively occurs, the number of carriers increases rapidly, creating a large current. This is the phenomenon called an avalanche breakdown. In a semiconductor that is applied an electric field of a magnitude just below the avalanche breakdown, an injection of carriers, even in a small number, can produce a large number of new carriers through the impact ionizations, resulting in a sudden increase in current. That is, a large current can be obtained even with an injection of a small number of carriers. This is a principle by which amplification is accomplished during the avalanche breakdown. The avalanche photodiode uses photo-induced carriers produced by an optical absorption for the carrier injection that triggers this phenomenon.
As well known, an important factor in terms of a high-speed response of the avalanche photodiode is the ionization rate ratio. The more the ionization rate ratio is away from unity, the better the performance of the avalanche photodiode becomes. Conversely, as the ionization rate ratio approaches unity, the amplification rate at high speed deteriorates, making it impossible to produce an avalanche photodiode with a good performance. Since infrared light is used in a high-speed optical communication, the fabrication of the light receiving device has so far used compound semiconductors, such as InP and InGaAs. However, the ionization rate ratio of InP, a typical compound semiconductor used in optical communication, is 0.5, relatively close to unity. Even with InAlAs the ionization rate ratio is 4 or 5 at most. Thus, an applicable frequency is about 10 GHz at most. As a result, a satisfactory performance cannot be obtained for high-speed devices of 40 GHz or higher.
On the other hand, Si has a very large ionization rate ratio ranging from 10 to more than 100 and thus can make a fast, highly sensitive avalanche photodiode. However, since Si cannot absorb light in an infrared frequency range used for optical communication, Si has not been able to be used for optical communication.
To overcome this drawback of the Si avalanche photodiode, an attempt has been made to combine Si with a compound semiconductor that has a sensitivity in the infrared range. For example, epitaxially growing a compound semiconductor on Si has been explored for a couple of decades now. However, no crystal with a satisfactory quality has been realized for practical use.
An example method for alleviating this quality problem of such a compound semiconductor on Si is disclosed in U.S. Pat. No. 6,384,462B1, which is briefly explained with reference to FIG. 2. In this patent, an avalanche photodiode is formed by directly fusing a Si multiplication layer 23 onto an InGaAs layer 22 epitaxially grown on a compound semiconductor substrate 21, as shown in FIG. 2. Further, by using ion implantation and diffusion techniques, a contact layer 24 and a guard ring 25 are formed. The use of the fusing technique keeps the crystalline structure of both the compound semiconductor and Si intact, so a high quality light receiving device can be obtained.
In the structure described above, however, the Si multiplication layer is directly fused at elevated temperatures to the InGaAs layer of a low carrier concentration that absorbs optical signal. Normally, on an interface of a junction between the InGaAs layer and the Si multiplication layer, there are many impurities including oxides. These impurities infiltrate into the InGaAs layer near the junction during the fusing process. As a result, the carrier concentration in the InGaAs layer near the junction increases, resulting in a high electric field being applied. The InGaAs layer has a narrow bandgap, so that when it is applied a high electric field, a dark current increases, degrading the sensitivity down to a level not suitable for practical use. In fact, in a device which has a Si multiplication layer directly fused to an InGaAs layer, the dark current exceeds a microampere, making the sensitivity of the device three or more orders of magnitude worse than those of conventional avalanche photodiodes in practical use. Further, a high electric field gives rise to a problem of causing an avalanche breakdown even in the InGaAs layer and thus degrading a high-speed response.
An object of the present invention is to provide an avalanche photodiode having a low dark current, a high sensitivity and a high speed and made of a combination of a compound semiconductor and Si, and to provide a method of manufacturing the same.